Synthesis of the Unknown Indeno[1,2‐a]fluorene Regioisomer: Crystallographic Characterization of Its Dianion

Abstract

Of the five possible indenofluorene regioisomers, examples of afully conjugated indeno[1,2-a]fluorene scaffold have so far remained elusive.This work reports the preparation and characterization of 7,12-dimesitylindeno[1,2-a]fluorene as ahighly reactive species.Experimental and computational data support the notion of am olecule with pronounced diradical character that exists in at riplet ground state.A ss uch, both NICS and ACID calculations suggest that the indeno[1,2a]fluorene scaffold is weakly Baird aromatic.Reduction of the unstable red solid with Cs metal produces the dianion of the title compound, from which single crystals could be obtained and X-raydata acquired, thus fully corroborating the proposed indeno[1,2-a]fluorene hydrocarbon core.

Azoniadibenzo[a,j]phenalenide: A Polycyclic Zwitterion with Singlet Biradical Character

“…With regard to the observation of thermallyexcited triplet states,v ariable-temperature 1 HNMR measurements showed broadening of the signals of H2, H8, and H10, where the triplet state has large spin densities,a t elevated temperatures (Figures 3a nd 4f), indicating that 3b can be thermally excited to the triplet state.F urther experimental evidence along this line was obtained by ESR measurements of the powdered sample of 3b,w hich gave signals of triplet species at 470 K( Figure 6). Thez ero-field splitting parameters were determined to be j D j= 20.2 mT, j E j= 2.1 mT.T he D value is consistent with the reported values of indenofluorene derivatives having an m-quinodimethane moiety, [34,35] supporting the observation of the triplet state of 3b (see Figure S20). Theintensity of the triplet signal decreased by lowering the temperature (see Figure S19), and the singlet-triplet energy gap was estimated to be À4400 K (À8.7 kcal mol À1 ), which is consistent with the calculated DE S-T of 3b (À6.9 kcal mol À1 ).…”

Stable Quadruple Helical Tetraradicaloid with Thermally Induced Intramolecular Magnetic Switching

“…[1][2][3][4] Motivated by these findings, incorporation of quinoidal substructures into an π-system has evolved as an important strategy toward molecules containing weakly paired or unpaired π-electrons. [5][6][7] This quinoidal strategy represents a simple approach for tuning a typical molecule with a closed-shell structure to an open-shell radicaloid, exemplified in indenofluorenes, [8][9][10][11] bisphenalenyls, 12,13 zethrenes, [14][15][16][17][18] quinoidal rylenes, [19][20][21][22] conjugated macrocycles, [23][24][25][26][27][28][29][30] and other motifs. [31][32][33][34][35][36][37][38][39] The generation of a radical character involves spatial reorganization of frontier π electrons, derived from re-aromatization of quinoidal substructures into aromatic diradical/polyradical resonance structures.…”

On-Surface Synthesis of Indenofluorene Polymers by Oxidative Five-Membered Ring Formation

“…[24][25][26] However, the inherent reactivity of indenofluorenes prohibited their practical application and also did not allow their preparation without bulky protecting groups at reactive positions. [27][28][29][30][31] To this end, on-surface synthesis has proven to be a very powerful method to create highly reactive structures, as represented by zigzag GNRs 8 as well as higher anthenes 32 and acenes. 33 Nevertheless, there are only very few reports discussing the on-surface formation of five-membered rings, which have always been embedded in fluoranthene subunits.…”